Winterkill in Alfalfa [description of common causes of stand loss]
- Frost Heaving: can be found when winter is extremely cold or open. Frost heaving is more likely to occur in heavier soils. A couple of actions kill plants and damage stands in this scenario: a period of alternating freezing/thawing cycles can cause the roots to sheer off [2” – 8” depth] or force the crown above the soil line [up to 6”]. The result is a stand that appears to “dry out” or “dry up” and die.
- Extreme low temperature: root and crown tissue can be terminated if the soil temperature drops below 15°F. Usually soil temperature reaches these levels due to lack of insulating snow cover.
- Ice sheeting: happens when rain or meltwater accumulates and freezes in a solid layer above the crowns. Through respiration and because of the ice layer, a high amount of carbon dioxide and ethylene accumulation occurs; combine this with the lack of oxygen [anoxia] and the result is plant death.
- Carbohydrate depletion: Plants “run out of fuel”. This can happen because of awkward timing of the final cut from the previous year or because of an extended winter and late break in dormancy. Another reason for running out of fuel is due to a false spring – or a false start to spring. Extended early warm-up and premature break in dormancy may cause crown buds to develop new shoots and as temperatures return to average, the shoots are frozen/terminated. New crown buds must form again [IF] there is enough stored carbohydrate (fuel). If carbohydrate root reserves are depleted, the plant may begin regrowth but seemingly runs out of fuel and withers until death.
What About 2017? Any of the aforementioned causes of death can be found in the upper Midwest as in any other year, but carbohydrate depletion is of particular concern in Northeast Iowa and Southwest Wisconsin. Temperatures have been unseasonably warm, especially in the middle of February. In some fields, a “false start” has occurred and we broke dormancy. This is something we will need to monitor closely as dormancy officially breaks and regrowth occurs.
What can we do?
- Determine what fields will be productive and what fields will not.
a. Field Productivity Ranking: number of viable plants/ft₂
and viable stems/ft
b. Crown and root health evaluation
c. Rotational restrictions, end-use assessment
Make a plan: determine whether to tear it up, keep it, or wait to make a decision [see how the plants regrow].